The legacy version is fully usable for watching videos, but expect a few limitations:
How to Download YouTube on iOS 12.5.7 (Older Devices: iPhone 5s, 6, iPad Mini 2) youtube ios 12.5 7 download
Google still supports a legacy version of YouTube for iOS 12. The current modern YouTube app requires iOS 14 or later, but when you try to download it on iOS 12.5.7, the App Store will automatically offer the last compatible version (typically YouTube 17.x or similar). The legacy version is fully usable for watching
If you have previously downloaded YouTube on your Apple ID, the App Store may offer you an older, compatible version. youtube ios 12.5 7 download
“This application requires iOS 14.0 or later.”